A conventional solar cell with a p-type (p-doped) silicon base has an n-type (n-doped) emitter in the form of an n-type diffusion layer on its front-side. This conventional silicon solar cell structure uses a negative electrode to contact the front-side, i.e. the sun side, of the cell and a positive electrode on the back-side. It is well known that radiation of an appropriate wavelength falling on a p-n junction of a semiconductor serves as a source of external energy to generate electron-hole pairs. The potential difference that exists at a p-n junction causes holes and electrons to move across the junction in opposite directions, thereby giving rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metallized, i.e., provided with metal electrodes which are electrically conductive. Typically, the front-side metallization is in the form of a so-called H pattern, i.e. in the form of a grid cathode comprising thin parallel finger lines (collector lines) and busbars intersecting the finger lines at right angles, whereas the back-side metallization is an aluminum anode in electric connection with silver or silver/aluminum busbars or tabs. The photoelectric current is collected by means of these two electrodes.
Alternatively, a reverse solar cell structure with an n-type silicon base is also known. This cell has a front p-type silicon surface (front p-type emitter) with a positive electrode on the front-side and a negative electrode to contact the back-side of the cell. Solar cells with n-type silicon bases (n-type silicon solar cells) can in theory produce higher efficiency gains compared to solar cells with p-type silicon bases owing to the reduced recombination velocity of electrons in the n-doped silicon.
As in the case of the conventional silicon solar cells, MWT silicon solar cells can be produced as MWT silicon solar cells having a p-type silicon base or, in the alternative, as MWT silicon solar cells having an n-type silicon base. As in conventional solar cells, the emitter of a MWT solar cell is typically covered with a dielectric passivation layer which serves as an antireflective coating (ARC) layer. However, MWT silicon solar cells have a cell design different than that of the conventional solar cells. The front-side electrodes of conventional solar cells reduce the effective photosensitive area available on the front-side of the solar cell and thereby reduce performance of the solar cell. MWT solar cells have both electrodes on the back-side of the solar cell. This is accomplished by drilling, e.g., with a laser, small holes that form vias between the front-side and the back-side of the cell.
The front-side of the MWT silicon solar cell is provided with a front-side metallization in the form of thin conductive metal collector lines which are arranged in a pattern typical for MWT silicon solar cells, e.g., in a grid- or web-like pattern or as thin parallel finger lines. The collector lines are applied from a conductive metal paste having fire-through capability. After drying, the collector lines are fired through the front-side dielectric passivation layer thus making contact with the front surface of the silicon substrate. The term “metal paste having fire-through capability” means a metal paste which etches and penetrates through (fires through) a passivation or ARC layer during firing thus making electrical contact with the surface of the silicon substrate.
The inside of the holes and, if present, the narrow rim around the front-edges of the holes, i.e., the diffusion layer not covered with the dielectric passivation layer, is provided with a metallization either in the form of a conductive metal layer on the sides of the hole or in the form of a conductive metal plug that completely fills the hole with conductive metal. The terminals of the collector lines overlap with the metallizations of the holes and are thus electrically connected therewith. The collector lines are applied from a conductive metal paste having fire-through capability. The metallizations of the holes are typically applied from a conductive metal paste and then fired. The metallizations of the holes serve as emitter contacts and form back-side electrodes connected to the emitter or electrically contact other metal deposits which serve as the back-side electrodes connected to the emitter.
The back-side of a MWT silicon solar cell also has the electrodes directly connected to the silicon base. These electrodes are electrically insulated from the metallizations of the holes and the emitter electrodes. The photoelectric current of the MWT silicon solar cell flows through these two different back-side electrodes, i.e., those connected to the emitter and those connected to the base.
Firing is typically carried out in a belt furnace for a period of several minutes to tens of minutes with the wafer reaching a peak temperature in the range of 550° C. to 900° C.
The efficiency of the MWT solar cells is improved since the emitter electrode is located on the back-side and thereby reduces shadowing of the photosensitive area available on the front-side of the solar cell. In addition the emitter electrodes can be larger in size and thereby reduce ohmic losses and all electrical connections are made on the back-side.
When producing a MWT solar cell there is a need for a conductive paste that results in a metalized hole that: (1) has sufficiently low series resistance between the collector lines and the emitter electrode, (2) has good adhesion to the sides of the hole and to the silicon on the backside of the solar cell and (3) has sufficiently high shunting resistance to prevent deleterious electrical connection between portions of the cell, i.e., the emitter and the base.